The Syrian proxy war is heating up, especially between the US and Russia.

On Tuesday, Bloomberg News reported that US and Kurdish forces in Syria had killed 200 fighters last week, including many Russian contract soldiers, according to several Russian sources and an American official.

The Russians, who were fighting on the side of embattled Syrian leader Bashar al Assad, attacked a military base in the Deir ez-Zor region of Syria on February 7, according to the Bloomberg report. US forces stationed with Syrian Democratic forces troops on the base responded with artillery fire and air strikes.

Ryan Dillon, a spokesperson for United States Central Command, confirmed in an email that that there was a raid conducted by pro-Assad forces, but would not confirm that any Russian fighters had been killed. US coalition forces were in “regular communication with Russian counterparts before, during and after the thwarted, unprovoked attack,” he wrote. “Russian officials assured Coalition officials they would not engage Coalition forces in the vicinity.”

Russian officials said that no Russian forces were involved in the attack, according to the New York Times.

Adrian Rankine-Galloway, a spokesperson for the US Department of Defense, said that officials believed roughly 100 people were killed in the raid, though said he couldn’t speculate about their nationality, while Bloomberg reported that the number dead was closer to 200. The New York Times said that “dozens” of Russians were believed to be killed.

CIA Director Mike Pompeo also refused to confirm specifics when asked whether the US was responsible for killing Russian contract soldiers in Syria during a hearing on Tuesday, but said, “We have seen in multiple instances foreign forces using mercenaries in battles that will begin to approach the United States.”

This incident comes at an odd time for the two countries. The Trump administration is under investigation for alleged collusion with Moscow during the US presidential campaign to tilt the election in Trump’s favor, and a Senate committee found that Russia had acted to influence the 2016 US presidential election.

President Trump has said that Russia constitutes one of its top national security threats, but has also cultivated a cozy relationship with Russian leader Vladimir Putin. It’s unclear what effect this incident, if true, will have on Russia-US relations. Putin and Trump spoke by phone on Monday, but reportedly did not discuss the Syria incident.

Syria’s conflict has spiraled into a proxy war with a confusing array of players

The report of Russian mercenaries being killed by US-backed forces is just the latest scuffle in Syria between various countries and regional actors.

Last week, Israel captured an Iranian drone entering Israeli airspace from Syria. As Vox’s Alex Ward writes, Israel responded by sending eight fighter jets back into Syria to attack the area from which the drone launched, near the city of Palmyra. Syria retaliated with anti-aircraft missiles, downing an Israeli F-16 jet. No one was killed, but the pilot was severely injured.

It was also a symbolic blow to perceptions of Israel in the region, the New York Times reports: The last time an Israeli aircraft was brought down by enemy fire was three decades ago.

Israel responded aggressively with several more strikes on known Iranian targets in Syria, and said it was “fully prepared for further action.” This latest incident has led experts to speculate about whether there is an Iranian-Israeli war on the horizon.

Dan Shapiro, a former US ambassador to Israel and a fellow at Tel Aviv University’s Institute for National Security Studies wrote in an email on Tuesday that a clash could happen, but wasn’t inevitable. “At the moment, both sides [Iran and Israel] seem to want to avoid further escalation,” he wrote.

The Syrian conflict, which began as an uprising against Bashar al Assad’s government during the Arab Spring in 2011, has ravaged the country and turned into a 16-year war. Similar to Lebanon’s civil war a few decades earlier, regional powers have seized on the unrest to wage their own proxy wars, and sometimes waded in directly.

Troops from Russia, Turkey, and the US are currently on the ground in Syria, in addition to ISIS fighters and other factions, like the Iranian-backed Lebanese militant group Hezbollah.

In the past two weeks, in addition to the Iranian drone incident and the Israeli strikes, Kurdish fighters in Syria shot down a Turkish helicopter that was attacking them last week, killing two Turkish soldiers on board, and Syrian rebels shot down a Russian fighter jet.